In today's vehicles that operate with internal combustion engines, catalysts are used to treat the exhaust gases in order to reduce emissions of certain pollutants. When the vehicle is new, the catalyst will generally operate within its design parameters. But catalyst aging, contaminates, and other factors may cause the effectiveness of the catalyst to be reduced.
In order to monitor the catalyst deterioration, some have suggested locating a pair of oxygen sensors in the exhaust stream, one upstream of the catalyst and one downstream of the catalyst. By comparing the readings from the two sensors, it is proposed that the effectiveness of the catalyst can be determined. But due to various factors that can affect the readings of the two sensors over long term use, and due to the limited accuracy that some methods of determining the deterioration have depending upon when and how the readings are taken, the actual determination of the amount of catalyst deterioration may become inaccurate over time. Thus, it is desirable to have a system for monitoring catalyst deterioration that is initially accurate and will remain accurate over a long period of time.